In recent years, attempts have been made actively to improve the energy efficiency of the power elements such as a power diode. Examples of the power diodes include Schottky barrier diodes (SBD) for obtaining rectifying properties. Since the power elements made of silicon materials are approaching the theoretical limit, the search for other materials that are more suitable for the power elements has been actively conducted.
It has been shown, in theory, that the semiconductor materials having a wide band gap and which were regarded conventionally as insulating materials are suitable as power elements (Non-Patent Document 1). Examples of the semiconductor materials having a wide band gap include metal nitrides such as gallium nitride (GaN) having a band gap of 3.4 eV, and AlN having a band gap of 6.2 eV. At present, power elements using GaN have been researched and developed actively. In addition, there are also reported cases of diodes using AlGaN (Non-Patent Document 2).
There is a reported example of aluminum nitride (AlN) diode that emits ultraviolet light with the shortest wavelength of 210 nm (Non-Patent Document 3). However, the AlN used in the diode is one formed as a thin film on a SiC substrate through heteroepitaxial growth, and is not a bulk single crystal of good quality. Further, the AlN diode is a p-n junction diode, and is not suitable for power diodes, since the ON resistance theoretically increases as compared with the Schottky barrier diode which is a power diode.
Production of high quality AlN single crystals is difficult and the device integration thereof is difficult, and thus the AlN Schottky diode operation has not been conventionally confirmed. For this reason, AlN has not attracted attention as a power element material until now.
It has been reported that it is possible to grow a large AlN single crystal on a SiC substrate by a sublimation method (Non-Patent Document 4). However, the AlN single crystal has not been reported as having semiconductor properties which may be used to form a Schottky barrier diode.